Most professional radiocommunication systems offer a functionality called the direct mode or walkie-talkie mode, which allows mobile radio terminals to communicate with one another outside of the coverage of a network.
This functionality has several uses: it makes it possible to make up for insufficient coverage of the network; it ensures continuity of service in the event of failure or of destruction of the fixed installations of the network; finally, it makes it possible to carry out network independent operations when the required.
The functions available in direct mode are in general more restricted than those available in network mode. One of the functions which is most often absent is the terminal standby function.
The standby function consists, for a terminal which is not communicating, in placing itself from time to time in a receive state so as to examine the state of the channel and to detect any calls relevant to it. For the remainder of the time, the terminal is sleeping so as to limit its energy consumption.
In former analog systems, the absence of a standby function did not present serious drawbacks. Indeed, the consumption of the reception part of the terminal in the absence of any signal was relatively small. Moreover, most of the terminals were automobile mobile terminals whose consumption was not critical.
The digital systems have greater consumption in the reception part, due to the necessary digital processing. With the development of handheld terminals, this makes terminal standby extremely useful.
In the network mode, the infrastructure controls the instants at which the terminals are woken up since it fixes the temporal structure for use of the channels. Synchronization of standby is therefore easily achieved. However, in direct mode, a form of cooperation must be set up between the terminals since the infrastructure may be out of range or nonexistent. Most of the standby mechanisms used in direct mode generally lead to a loss of the start of the communication or to a relatively long setup time.
European patent 0 786 174 describes a system in which base stations of the network supervise the standby function of the terminals both in respect of the radio resources of the network and in respect of the direct mode. This approach is suitable only in the particular case where the terminals are within range of base stations. This case is not the most frequent.
An object of the present invention is to provide an efficient standby mechanism in direct mode, i.e. one which offers a relatively large amount of standby and guarantees a relatively short communication setup time.
It is also desirable for the mechanism to be well adapted to various modes of communication setup: mode without acknowledgment (the most common in direct mode), or modes offering a possibility of acknowledgment, for example by going off-hook after ringing.